Electrowetting on dielectric (EWOD) is a well known technique for manipulating droplets of fluid by application of an electric field. It is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007) 3:245-281).
FIG. 1 shows a part of an EWOD device in cross section. The device includes a lower substrate 72 the uppermost layer of which is formed from a conductive material which is patterned so that a plurality of conductive electrodes 38 (e.g., 38A and 38B in FIG. 4) are realised. These may be termed the EW drive elements. A droplet 4, consisting of an ionic material is constrained in a plane between the lower substrate 72 and a top substrate 36. A suitable gap between the two substrates may be realised by means of a spacer 32, and a non-ionic liquid 34 (e.g. oil) may be used to occupy the volume not occupied by the liquid droplet 4. An insulator layer 20 disposed upon the lower substrate 72 separates the conductive electrodes 38A, 38B from a hydrophobic surface of a hydrophobic layer 16 upon which the liquid droplet 4 sits with a contact angle 6 represented by θ. On the top substrate 36 is another hydrophobic layer 26 with which the liquid droplet 4 may come into contact. Interposed between the top substrate 36 and the hydrophobic layer 26 is a top substrate electrode 28. In operation, voltages, termed the EW drive voltages, (e.g. VT, V0 and V00) may be externally applied to different electrodes (e.g. drive element electrodes 28, 38A and 38B, respectively). The hydrophobicity of the hydrophobic surface of the layer 16 can be thus controlled, thus facilitating droplet movement in the lateral plane between the two substrates 72 and 36.
U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) discloses a passive matrix EWOD device for moving droplets through an array.
U.S. Pat. No. 6,911,132 (Pamula et al, issued Jun. 28, 2005) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions.
U.S. Pat. No. 6,565,727 further discloses methods for other droplet operations including the splitting and merging of droplets and the mixing together of droplets of different materials. In general the voltages required to perform typical droplet operations are relatively high. Values in the range 20 volts (V)-60V are quoted in the prior art (e.g. U.S. Pat. No. 7,329,545 (Pamula et al., issued Feb. 12, 2008)). The value required depends principally on the technology used to create the insulator and hydrophobic layers.
In practise the creation of a suitable insulator layer 20, of sufficiently high quality to facilitate mass production of EWOD devices, can often prove challenging. A suitable, high quality insulator layer must be resilient to electrical breakdown and suitably non-porousso such that ions from the liquid droplet 4 are unable to traverse through the insulator layer under the action of the electric field. In practise it is difficult to fabricate insulator layers where this is always achieved over every part of every electrode 38. Typically insulator layers may suffer from “insulator pinhole defects” in the insulator layer at a few discrete locations within the device. At these locations dielectric breakdown may then occur within the device. Often, the occurrence of an insulator pinhole defect may only become apparent when a liquid droplet 4 comes to reside at the location of the insulator pinhole defect and when the electrode in that location is an actuated state such that an electric field is developed across the insulator layer. Under these circumstances mobile ions from the liquid droplet 4 may traverse through the liquid leading to electrolysis of the liquid, usually at the interface between the liquid droplet 4 and the bottom substrate 72. The occurrence of electrolysis may cause damage to the device and may compromise the chemistry of the liquid droplet 4. In general in EWOD device implementations it is very difficult to determine if, when and where electrolysis due to insulator pinhole defects occurs without viewing the device optically (e.g. through a microscope).
“Electrowetting droplet microfluidics on a single planar surface”, Microfluid Nanofluid (2006) 2:435-446 (Cooney et al.) describes how an EWOD device may be protected from damage due to electrolysis at the site of insulator pinhole defects by current limiting the power supply used to supply the EWOD drive voltage to 1 microampere (μA).
U.S. Pat. No. 7,163,612 (J. Sterling et al.; issued Jan. 16, 2007) describes how TFT based electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in AM display technologies.
The approach of U.S. Pat. No. 7,163,612 may be termed “Active Matrix Electrowetting on Dielectric” (AM-EWOD). There are several advantages in using TFT based electronics to control an EWOD array, namely:                Driver circuits can be integrated onto the AM-EWOD array substrate.        TFT-based electronics are well suited to the AM-EWOD application. They are cheap to produce so that relatively large substrate areas can be produced at relatively low cost        TFTs fabricated in standard processes can be designed to operate at much higher voltages than transistors fabricated in standard CMOS processes. This is significant since many EWOD technologies require EWOD actuation voltages in excess of 20V to be applied.        
A disadvantage of U.S. Pat. No. 7,163,612 (J. Sterling et al.; issued Jan. 16, 2007) is that it does not disclose any circuit embodiments for realising the TFT backplane of the AM-EWOD.
EP2404675 (Hadwen et al.; published Jan. 11, 2012) describes array element circuits for an AM-EWOD device. Various methods are described for programming and applying an EWOD actuation voltage to the EWOD drive electrode. The voltage write function described includes a memory element of standard means, for example based on Dynamic RAM (DRAM) or Static RAM (SRAM) and input lines for programming the array element. EP2404675 also describes how an impedance sensing function can be incorporated into the array element.
A potential disadvantage of AM-EWOD in general is that the EWOD actuation voltage that can be supplied is limited to the maximum voltage rating of the TFTs. High voltage operation of TFTs may result in device degradation or failure as is well known. Therefore in order to achieve sufficient EWOD actuation, the total capacitance of the insulator layer and hydrophobic layer cannot be designed to be too small. This in effect constrains the maximum thickness of insulator layer that can be used. Therefore attempting to improve the device reliabililty by the use of thicker insulator layers (which may have a lower occurrence of insulator pinhole defects) is impractical because the drive voltage cannot be increased accordingly due to the operating limit of the TFTs.
U.S. Pat. No. 4,685,086 (Tran, published Aug. 4, 1987) describes a circuit for detecting a short circuit in an SRAM memory cell. This includes means for connecting the nodes of the memory cell to the gates of pulldown transistors. The pulldown transistors perform a level shifting function to produce a voltage pattern that is dependent on whether the memory cell is functioning correctly or not.